Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 505
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-Section 50
-Paragraph 3
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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

+ References

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Securities - Amortized Cost, Estimated Fair Value and Credit Rating of Corporate Debt Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		$ 905,496	$ 851,903
Estimated Fair Value	$ 896,812	$ 832,815
Chase Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	10,000
Estimated Fair Value	$ 9,150
Credit Rating Moody’s/ S&P	Baa1/BBB-
Wells Fargo Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,606
Credit Rating Moody’s/ S&P	A1/BBB
Huntington Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,450
Credit Rating Moody’s/ S&P	Baa2/BB+
Keycorp Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,525
Credit Rating Moody’s/ S&P	Baa2/BB+
PNC Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,605
Credit Rating Moody’s/ S&P	Baa1/BBB-
State Street Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,625
Credit Rating Moody’s/ S&P	A3/BBB
SunTrust Capital
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 5,000
Estimated Fair Value	$ 4,625
Credit Rating Moody’s/ S&P	Not Rated/BB+
Celgene
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,518
Estimated Fair Value	$ 1,501
Credit Rating Moody’s/ S&P	Baa2/BBB+
Southern Company
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,512
Estimated Fair Value	$ 1,500
Credit Rating Moody’s/ S&P	Baa2/BBB+
BB&T
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,510
Estimated Fair Value	$ 1,499
Credit Rating Moody’s/ S&P	A2/A-
AT&T Inc.
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,507
Estimated Fair Value	$ 1,494
Credit Rating Moody’s/ S&P	Baa2/BBB
Ryder
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,504
Estimated Fair Value	$ 1,501
Credit Rating Moody’s/ S&P	Baa1/BBB+
HAL
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 460
Estimated Fair Value	$ 460
Credit Rating Moody’s/ S&P	Baa1/A-
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 48,011
Estimated Fair Value	$ 44,541